Photocatalysts play an important role as a so-called environmental catalyst which eliminate nitrogen oxides (NOx) and harmful organic compounds in the atmospheric air, such as formaldehyde, chlorinated hydrocarbons, dioxins and the like, or harmful organic materials present in daily life water, various types of household effluents and industrial effluents to clean the environment.
Titanium dioxide is widely used as this photocatalyst in respect of the most stable one with almost no toxicity against living organisms. When this titanium dioxide is exposed to near UV radiation around 400 nm, an electron in the filled band is excited to the conduction band to cause charge separation. The resultant charge site serves as a source to generate a hydroxyl radical or a superoxide anion which decomposes environmental pollutants, such as organic halides and nitrogen oxides, by its strong oxidizing action.
However, titanium dioxide is photocatalytically active only in a wavelength range around 400 nm, and not photocatalytically active in other wavelength ranges, and therefore naturally subject to a limited range of applications.
For that purpose, photocatalysts have been proposed, such as a composite of titanium dioxide and an inorganic porous material, e.g., activated carbon, high-silica zeolite, silica gel, sepiolite, bentonite, magnesium sulfate and others (JP2001-276194A); a visible light-sensitive photocatalyst comprising a titanium dioxide film having a very thin layer of N-doped TiO2 formed on the surface layer (JP2003-265966A); a photocatalytic composition comprising a visible light-sensitive photocatalyst and a photocatalyst having a specific surface area larger than the said visible light-sensitive photocatalyst (JP2003-340289A) and the like.
Furthermore, alternative photocatalysts free of titanium dioxide have been proposed, such as a composite photocatalyst for hydrogen generation comprising cadmium sulfide and a sulfide of a different metal (JP2001-239164A); a semiconductor photocatalyst having semiconductor particles encapsulated with a polymer (JP10-310401A); a photocatalyst comprising layered composite metal oxide including interlayer cadmium sulfide (JP2001-157843A) and the like. However, none of the photocatalysts can provide so high a conversion rate as to be feasible.
On the other hand, methods using a silicon oxide as a photocatalyst have been known such as, for example, a method for photooxidation of ethylene with silica (Studies in Surface Science and Catalysts, vol. 130, 2000, p. 1955-1960), a method for epoxidation of propylene with gaseous oxygen in the presence of silica or manganese-loaded silica (J. Catalysts, vol. 171, 1997, p. 351-357) and others. However, these methods not only have to require an ultrahigh-pressure mercury lamp as a light source but provide a low conversion rate of at most 30%, so that they are far from feasible.
Under such circumstances, the inventors have already proposed a photocatalyst comprising fused quartz treated with a hydrohalogeno acid (JP2003-83950) and a method of eliminating nitrogen oxides by photooxidation with the same photocatalyst (JP2003-83951). The photocatalyst has advantages of being effective for radiation in a wider range of wavelengths compared with conventional silicon oxide-based photocatalysts and producing nitric acid at a much higher rate than titanium dioxide-based photocatalysts, but disadvantageously it is difficult to obtain due to the unusual base material that is fused quartz, and does not decompose at a high rate harmful substances except nitrogen oxides, such as toluene, acetaldehyde, ethanedithiol and others. Accordingly, the photocatalyst is not always satisfactory yet for practical use.